Is it legal for a vendor to put kerosene in a plastic gasoline can?
(I assume the opposite is illegal, but I don't know about this.)

Is there any other reason not to put in in one?

If after I've emptied all the kerosene and want to use it for
gasoline, if there's a little bit of kerosene still in it, will that
be a problem for a lawn mower or a car?

I have a spare platic can and no room for a kerosene can, and I would
only need it for a few months anyhow.